The present disclosure relates to a flight control system, and more particularly to a flight control system auto-trim logic which facilities use of unique trim controllers.
A rotary-wing aircraft flight control system typically employs a displacement controller, such as a center cyclic stick, whose position provides an indication to the pilot of the angular disposition or attitude of the rotor. With a displacement controller, there is full correlation between the position of the controller and control surface command. Emergent generations of fly-by-wire (FBW) aircraft such as rotary-wing aircraft provide high levels of augmentation. The pilot-vehicle interface within a rotary-wing aircraft environment often utilize “unique trim” controllers. A unique-trim controller does not produce a fixed amount of output for a given pilot input; instead the unique-trim controller integrates the input over time and produces an output that is a combination of the controller displacement and the controller displacement time period. The direct correlation between controller position and control surface command that is present with a displacement controller is absent with a unique-trim controller.
The unique-trim controllers always return to a null or center position when the aircrew does not apply a force to the controller. This operates effectively in a fully augmented mode, where displacement from center commands a particular action, i.e. an attitude or a velocity change, and return to the center position commands a hold function, i.e. attitude or velocity hold.
One disadvantage of unique trim controllers may become apparent during aircraft state sensor failure. During such failures it is no longer possible to provide the full level of augmentation and the flight control system reverts to some combination of rate feedback within a direct rotor control mode. In the direct rotor control mode, unique trim may complicate flight control since the actual trim (equilibrium) point of the aircraft varies with aircraft states such as airspeed, gross weight, etc. This may result in the aircrew having to hold the controller stick against a centering force.
To alleviate this, prior FBW rotary-wing aircraft implemented a trim “follow-up” logic. Follow-up logic feeds selected—usually relatively low—frequency inputs from the controllers to integrators in each axis to slowly translate the flight control position toward the controller position. In this scheme, the aircrew will eventually return the controller to the center, while the flight control will remain in the new trimmed state. This operates well for pitch cyclic and for roll cyclic.
For yaw pedals, however, unique trim control with trim follow-up may result in an “unnatural” feel. In a direct mode, sometimes known as stick-to-head mode, the aircrew rolls the aircraft into a turn and applies pedal until a coordinated turn is established. With trim follow-up, the aircrew is forced to slowly reduce yaw pedal input during the coordinated turn since trim follow-up is always moving the tail rotor pitch towards the applied yaw pedal. This may increase workload during coordinated turns in the direct mode.